Phase 13 v1 + E5-2 retest: Both NEUTRAL, freeze as research boxes

Phase 13 v1: Header Write Elimination (C7 preserve header)
- Verdict: NEUTRAL (+0.78%)
- Implementation: HAKMEM_TINY_C7_PRESERVE_HEADER ENV gate (default OFF)
- Makes C7 nextptr offset conditional (0→1 when enabled)
- 4-point matrix A/B test results:
  * Case A (baseline): 51.49M ops/s
  * Case B (WRITE_ONCE=1): 52.07M ops/s (+1.13%)
  * Case C (C7_PRESERVE=1): 51.36M ops/s (-0.26%)
  * Case D (both): 51.89M ops/s (+0.78% NEUTRAL)
- Action: Freeze as research box (default OFF, manual opt-in)

Phase 5 E5-2: Header Write-Once retest (promotion test)
- Verdict: NEUTRAL (+0.54%)
- Motivation: Phase 13 Case B showed +1.13%, re-tested with dedicated 20-run
- Results (20-run):
  * Case A (baseline): 51.10M ops/s
  * Case B (WRITE_ONCE=1): 51.37M ops/s (+0.54%)
- Previous test: +0.45% (consistent with NEUTRAL)
- Action: Keep as research box (default OFF, manual opt-in)

Key findings:
- Header write tax optimization shows consistent NEUTRAL results
- Neither Phase 13 v1 nor E5-2 reaches GO threshold (+1.0%)
- Both implemented as reversible ENV gates for future research

Files changed:
- New: core/box/tiny_c7_preserve_header_env_box.{c,h}
- Modified: core/box/tiny_layout_box.h (C7 offset conditional)
- Modified: core/tiny_nextptr.h, core/box/tiny_header_box.h (comments)
- Modified: core/bench_profile.h (refresh sync)
- Modified: Makefile (add new .o files)
- Modified: scripts/run_mixed_10_cleanenv.sh (add C7_PRESERVE ENV)
- Docs: PHASE13_*, PHASE5_E5_2_HEADER_WRITE_ONCE_* (design/results)

Next: Phase 14 (Pointer-chase reduction, tcache-style intrusive LIFO)

🤖 Generated with Claude Code

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

core/box/tiny_c7_preserve_header_env_box.c

@@ -0,0 +1,50 @@
+// ============================================================================
+// Phase 13 v1: Tiny C7 Preserve Header ENV Box (L0) - Implementation
+// ============================================================================
+
+#include "tiny_c7_preserve_header_env_box.h"
+#include <stdlib.h>
+#include <string.h>
+#include <stdio.h>
+#include <unistd.h>
+
+// ============================================================================
+// Global State
+// ============================================================================
+
+_Atomic int g_tiny_c7_preserve_header_enabled = -1;
+
+// ============================================================================
+// Init (Cold Path)
+// ============================================================================
+
+int tiny_c7_preserve_header_env_init(void) {
+    const char* env = getenv("HAKMEM_TINY_C7_PRESERVE_HEADER");
+    int enabled = 0;  // default: OFF (opt-in)
+
+    if (env && (env[0] == '1' || strcmp(env, "true") == 0 || strcmp(env, "TRUE") == 0)) {
+        enabled = 1;
+    }
+
+    // Cache result
+    atomic_store_explicit(&g_tiny_c7_preserve_header_enabled, enabled, memory_order_relaxed);
+
+    // Log once (stderr for immediate visibility)
+    if (enabled) {
+        const char msg[] = "[C7_PRESERVE_HEADER] enabled\n";
+        ssize_t w = write(2, msg, sizeof(msg) - 1);
+        (void)w;
+    }
+
+    return enabled;
+}
+
+// ============================================================================
+// Refresh (Cold Path, called from bench_profile)
+// ============================================================================
+
+void tiny_c7_preserve_header_env_refresh_from_env(void) {
+    // Reset to uninitialized state (-1)
+    // Next call to tiny_c7_preserve_header_enabled() will re-read ENV
+    atomic_store_explicit(&g_tiny_c7_preserve_header_enabled, -1, memory_order_relaxed);
+}

core/box/tiny_c7_preserve_header_env_box.h

@@ -0,0 +1,72 @@
+// ============================================================================
+// Phase 13 v1: Tiny C7 Preserve Header ENV Box (L0)
+// ============================================================================
+//
+// Purpose: ENV gate for C7 header-preserving freelist layout
+//
+// Design: docs/analysis/PHASE13_HEADER_WRITE_ELIMINATION_1_DESIGN.md
+// Instructions: docs/analysis/PHASE13_HEADER_WRITE_ELIMINATION_1_NEXT_INSTRUCTIONS.md
+//
+// Strategy:
+//   - C7 (1025-2048B) の freelist が header を壊さないようにする
+//   - nextptr offset を 0→1 に変更（header 1B をスキップ）
+//   - これにより alloc 時の header 再書き込みを削減できる
+//
+// ENV:
+//   HAKMEM_TINY_C7_PRESERVE_HEADER=0/1 (default: 0, opt-in)
+//
+// API:
+//   tiny_c7_preserve_header_enabled() -> int
+//   tiny_c7_preserve_header_env_refresh_from_env()
+//
+// Box Theory:
+//   - L0: This file (ENV gate,戻せる)
+//   - L1: tiny_layout_box.h (SSOT: tiny_nextptr_offset)
+//   - L2: tiny_nextptr.h, tiny_header_box.h (affected code)
+//
+// Safety:
+//   - ENV-gated (default OFF, opt-in)
+//   - Reversible (ENV toggle)
+//   - Minimal change (C7 offset 0→1 のみ)
+//
+// ============================================================================
+
+#ifndef TINY_C7_PRESERVE_HEADER_ENV_BOX_H
+#define TINY_C7_PRESERVE_HEADER_ENV_BOX_H
+
+#include <stdatomic.h>
+
+// ============================================================================
+// Global State (L0)
+// ============================================================================
+
+// Cached state: -1 (uninitialized), 0 (disabled), 1 (enabled)
+extern _Atomic int g_tiny_c7_preserve_header_enabled;
+
+// ============================================================================
+// Hot Inline API (L0)
+// ============================================================================
+
+// Check if C7 preserve header is enabled
+// Returns: 1 if enabled, 0 if disabled
+static inline int tiny_c7_preserve_header_enabled(void) {
+    int val = atomic_load_explicit(&g_tiny_c7_preserve_header_enabled, memory_order_relaxed);
+
+    if (__builtin_expect(val == -1, 0)) {
+        // Lazy init: read ENV once
+        extern int tiny_c7_preserve_header_env_init(void);
+        val = tiny_c7_preserve_header_env_init();
+    }
+
+    return val;
+}
+
+// ============================================================================
+// Cold API (L2)
+// ============================================================================
+
+// Refresh ENV cache (called from bench_profile after putenv)
+// Pattern: Same as Phase 8 (FREE_STATIC_ROUTE)
+extern void tiny_c7_preserve_header_env_refresh_from_env(void);
+
+#endif  // TINY_C7_PRESERVE_HEADER_ENV_BOX_H

core/box/tiny_header_box.h

@@ -41,13 +41,14 @@
 //
 // Returns:
 //   true  - C1-C6: Header preserved at offset 0, next at offset 1
-//   false - C0, C7: Header overwritten by next pointer at offset 0
+//   false - C0: Header overwritten by next pointer at offset 0
+//   Phase 13 v1: C7 returns false (default) or true (HAKMEM_TINY_C7_PRESERVE_HEADER=1)
 
 static inline bool tiny_class_preserves_header(int class_idx) {
 #if HAKMEM_TINY_HEADER_CLASSIDX
     // Delegate to tiny_layout_box.h specification (Single Source of Truth)
-    // next_off=0 → header overwritten (C0, C7)
-    // next_off=1 → header preserved (C1-C6)
+    // next_off=0 → header overwritten (C0, C7 default)
+    // next_off=1 → header preserved (C1-C6, C7 with HAKMEM_TINY_C7_PRESERVE_HEADER=1)
     return tiny_nextptr_offset(class_idx) != 0;
 #else
     // Headers disabled globally
@@ -87,7 +88,8 @@ static inline void tiny_header_write_if_preserved(void* base, int class_idx) {
 // ============================================================================
 //
 // Validates header ONLY if this class preserves headers.
-// For C0/C7, validation is impossible (next pointer is stored at offset 0).
+// For C0, validation is impossible (next pointer is stored at offset 0).
+// Phase 13 v1: C7 validation depends on HAKMEM_TINY_C7_PRESERVE_HEADER.
 //
 // Arguments:
 //   base      - BASE pointer (not user pointer)

core/box/tiny_layout_box.h

@@ -79,14 +79,29 @@ static inline size_t tiny_user_offset(int class_idx) {
 // Offset for storing the freelist next pointer inside a freed block.
 // This is distinct from tiny_user_offset():
 //   - User offset is always +1 in header mode.
-//   - Next offset is 0 for C0/C7 (cannot preserve header while free), else 1.
+//   - Next offset:
+//     - C0: always 0 (16B, cannot fit header+next)
+//     - C1-C6: always 1 (header-preserving)
+//     - C7: 0 (default) or 1 (Phase 13 v1: header-preserving)
 static inline size_t tiny_nextptr_offset(int class_idx) {
 #if HAKMEM_TINY_HEADERLESS
     (void)class_idx;
     return 0;
 #elif HAKMEM_TINY_HEADER_CLASSIDX
-    // Bit pattern: C0=0, C1-C6=1, C7=0 → 0b01111110 = 0x7E
-    return (0x7Eu >> ((unsigned)class_idx & 7u)) & 1u;
+    // Phase 13 v1: C7 preserve header gate
+    // Bit pattern (default): C0=0, C1-C6=1, C7=0 → 0b01111110 = 0x7E
+    // Bit pattern (C7 preserve): C0=0, C1-C7=1 → 0b11111110 = 0xFE
+    unsigned int base_pattern = 0x7Eu;  // default: C7 offset=0
+
+    // Phase 13 v1: Gate for C7 header-preserving layout
+    if (class_idx == 7) {
+        extern int tiny_c7_preserve_header_enabled(void);
+        if (tiny_c7_preserve_header_enabled()) {
+            base_pattern = 0xFEu;  // C7 offset=1 (header-preserving)
+        }
+    }
+
+    return (base_pattern >> ((unsigned)class_idx & 7u)) & 1u;
 #else
     (void)class_idx;
     return 0u;